U.S. patent number 5,172,410 [Application Number 07/515,067] was granted by the patent office on 1992-12-15 for conference telephone system and method.
This patent grant is currently assigned to Sound Control Technologies, Inc.. Invention is credited to Artner B. Chace.
United States Patent |
5,172,410 |
Chace |
December 15, 1992 |
Conference telephone system and method
Abstract
A speaker telephone comprising a microphone outputting an
electrical signal, and a loudspeaker is disclosed. An amplifier has
its output coupled to the loudspeaker. This amplifier has
non-inverting and inverting inputs. A switch having two positions
couples the microphone to the amplifier or couples the tuning
signal source to one of the inputs of the amplifier. A tuning
signal source and control circuit output a tuning signal.
Initiation means initiates a tuning sequence by causing the switch
to be in the second position and causing the tuning signal source
and control circuit to generate tuning signals. A coupling device
is connected to one of the inputs of the amplifier and adopted to
be connected to a telephone circuit. A hybrid voltage generator
circuit is responsive to the output of the microphone when the
switch couples the microphone to the amplifier to generate at its
output a cancellation signal which prevents transmitted voice
signals from appearing at the loudspeaker.
Inventors: |
Chace; Artner B. (Granby,
MA) |
Assignee: |
Sound Control Technologies,
Inc. (Norwalk, CT)
|
Family
ID: |
24049841 |
Appl.
No.: |
07/515,067 |
Filed: |
April 26, 1990 |
Current U.S.
Class: |
379/388.02;
379/30; 379/390.01; 379/390.04; 379/391; 379/394; 379/400 |
Current CPC
Class: |
H04M
1/6033 (20130101); H04M 9/08 (20130101) |
Current International
Class: |
H04M
1/60 (20060101); H04M 9/08 (20060101); H04M
001/00 () |
Field of
Search: |
;379/394,388,391,392,5,6,30,400,402,403,404 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Dwyer; James L.
Assistant Examiner: Shehata; M.
Attorney, Agent or Firm: Handal & Morofsky
Claims
I claim:
1. A speaker telephone, comprising:
(a) a microphone outputting an electrical signal;
(b) a loudspeaker;
(c) a tuning signal source and control circuit which outputs a
tuning signal;
(d) an amplifier whose output is coupled to said loudspeaker, said
amplifier having two inputs, one of said inputs being a
non-inverting input and the other of said inputs being an inverting
input;
(e) a switch having two positions, said switch coupling said
microphone to one of the inputs of said amplifier in a first of
said two positions and coupling said tuning signal source to said
one of the inputs of said amplifier in a second of said two
positions;
(f) initiation means for initiating a tuning sequence by causing
said switch to be in said second position and causing said tuning
signal source and control circuit to generate tuning signals;
(g) a coupling device connected to said one of the inputs of said
amplifier and adapted to be connected to a telephone circuit for
inputting signals from a telephone circuit to said amplifier to be
amplified by said amplifier and converted into audio signals by
said loudspeaker;
(h) a hybrid voltage generator circuit responsive to the output of
said microphone when said switch couples said microphone to said
amplifier to generate at its output a cancellation signal, said
cancellation signal being coupled to the other input of said
amplifier, said cancellation signal having a magnitude and phase
which results in substantially cancelling at least a portion of the
signals produced by said microphone from appearing at the output of
said amplifier; and
(i) a detector responsive to the output of said amplifier in
response to said tuning signals to generate an error signal, said
control circuit receiving said error signal and deriving control
signals in response thereto, said control signals being sent to
said hybrid circuit to control parameters of said hybrid circuit to
achieve cancellation of a substantial portion of the signal output
by said microphone at the output of said amplifier.
2. A speaker telephone as in claim 1, wherein said control circuit
is a microprocessor.
3. A speaker telephone as in claim 2, wherein said control circuit
provides synchronization signals to said detector, and said
detector is a synchronous detector.
4. A speaker telephone as in claim 3, wherein said synchronous
detector detects the real and imaginary components of said error
signal.
5. A speaker telephone as in claim 3, wherein said synchronous
detector detects the real and imaginary components of said error
signal.
6. A speaker telephone as in claim 5, wherein said hybrid voltage
generator circuit synthesizes the effect of resistive, capacitive
and inductive components of said telephone circuit.
7. A speaker telephone as in claim 6, wherein said hybrid voltage
generator circuit synthesizes the effect of lumped, resistive,
distributed, capacitive and inductive components of said telephone
circuit.
8. A speaker telephone as in claim 7, wherein said hybrid voltage
generator circuit comprises an adjustable resistive subsystem and a
plurality of RC networks, said RC networks being adjustably coupled
to said outputs of said hybrid circuit.
9. A speaker telephone as in claim 8, wherein said plurality of RC
networks comprise four RC networks, two of which four RC networks
are low pass networks and two of which four RC networks are high
pass networks.
10. A speaker telephone as in claim 9, wherein said control circuit
is a microprocessor.
11. A speaker telephone as in claim 10, wherein said control
circuit provides synchronization signals to said detector, and said
detector is a synchronous detector.
12. A speaker telephone as in claim 11, wherein said control
circuit provides synchronization signals to said detector, and said
detector is a synchronous detector.
13. A speaker telephone as in claim 12, wherein said microprocessor
circuit generates synchronization signals which are in phase and
90.degree. out of phase with said tuning signals.
14. A speaker telephone as in claim 13, wherein said tuning signals
occur at frequencies of about 300, 500, 800, 1300, 2000 and 3000
Hertz.
15. A speaker telephone as in claim 13, wherein said microprocessor
circuit terminates said tuning sequence if said error signal is
below a predetermined threshold.
16. A speaker telephone as in claim 14, wherein said circuit
terminates said tuning sequence if said error signal is below a
predetermined threshold.
17. A speaker telephone as in claim 1, wherein said microphone is
coupled to said amplifier through a resistance connected to one end
to said amplifier, said coupling device is a transformer comprising
two windings, one of said windings being connected to said
amplifier and said one end of said resistor and the other of said
windings being connected to said telephone circuit.
18. A speaker telephone as in claim 1, wherein said hybrid voltage
generator circuit comprises five segments, the first segment
generating a resistive component, the second and third segments
generating high frequency components, and the fourth and fifth
segments generating low frequency components.
19. A speaker telephone as in claim 18, wherein said second and
fourth segments generate real and imaginary components, and said
third and fifth components generate real or imaginary
components.
20. A speaker telephone as in claim 19, wherein said third and
fifth segments generate imaginary components.
Description
TECHNICAL FIELD
The present invention relates to circuits for providing duplex
telecommunication over telephone lines in the context of a
conference telephone system comprising a loudspeaker and a
microphone.
BACKGROUND
One very basic aspect of face-to-face human communication is the
ability of the two parties to both talk and be heard at the same
time. This aspect of face-to-face communication is quite important,
insofar as it enables parties to a conversation to interrupt each
other and thus build substantial efficiency into their
communication. It is also important in other respects, for example,
in the event that one party is beginning to speak about something
which may, perhaps unknown to him, be uncomfortable to a third
party to the conversation or otherwise destructive of the object of
the communication.
While the ability to interrupt the speech of another seems quite
natural, it is an aspect of face-to-face communication not found in
many electronic telecommunications systems. Indeed, the ability to
both speak and be heard at the same time presents technical
complications in most telecommunication systems. For example, if we
consider radio frequency carrier wave transmissions, if two parties
to a conversation transmit at the same time, the signals will
interfere with each other causing beat frequency oscillations,
feedback and the like. Systems which solve this problem and thus
allow the parties to the conversation to both speak and be heard at
the same time are referred to as duplex systems. This can be
achieved, for example, in the case of radio frequency
communication, by a pair of transmitters operating with respective
receivers at two different frequencies, one assigned to each of the
parties.
In contrast, until the introduction of speaker telephones,
virtually all telephones were duplex systems. Generally, the
operative portion of telephone systems during this period comprised
the series combination of a variable resistance carbon microphone
and an electromagnetic earphone. As the caller spoke over the line,
a diaphragm coupled to a carbon powder compartment in the
microphone caused successive compressions to be exerted against the
carbon powder in the compartment thus varying the electrical
resistance of the compartment. This in turn varied the current
passing through the series circuit resulting in a modulation of a
signal placed across the series combination. This modulated signal,
modulated at the actual frequency of the voice signal being
transmitted, was then sent over the telephone system to the
telephone set of the other participant to the telephone
conversation.
This system, which continues in use substantially unchanged from
the original instruments developed by Bell in the 1870's, as noted
above, inherently has a duplex characteristic. Duplex communication
is achieved because the audio frequencies involved do not cause
unacceptable interference with each other and because the gain of
the potential feedback loop between the carbon microphone and the
earphone is far less than one.
At the advent of speaker telephones, it became necessary to
introduce into the telephone instrument, an audio amplifier for
receiving audio signals from the telephone central office and
amplifying them to drive a loudspeaker. This immediately presented
the problem of preventing feedback between a microphone adjusted
for sensitivity to the voice of a person who is not speaking
directly into it while making the system unresponsive to audio
signals introduced into the environment by the loudspeaker. To
somewhat better understand this problem, it must be kept in mind
that the telephone is a two-wire system used to carry both the
transmitted and received signal. If the transmitted signal is thus
allowed to be amplified by the amplifier which amplifies the
received signal which is also carried on the same two wires,
ambient noise will be amplified and feedback oscillations are
likely to ensue at normal levels of speaker amplitude.
One approach to this problem was embodied in speaker phone systems
which included separate microphones and loudspeakers, both of which
had some directional characteristic designed to ensure that
information on loudspeaker would be loud enough for the telephone
use to hear while at the same time having less audio field strength
at some point where the microphone was placed. Likewise, the
solution involved a microphone whose sensitivity characteristic was
directed toward the mouth of the individual using the system with
minimal sensitivity in the vicinity of the speaker.
Thus, design objectives involved reducing the gain of the feedback
loop between the microphone and the speaker to less than one with
the volume control for the system set at a level which would allow
easy intelligibility of the signal.
Such an approach does not, in principle, provide a commercially
acceptable level of performance, as, for example, it imposes limits
on the location of the parties to the conversation. Moreover, the
provision of several microphones is required in order to achieved
good spatial separation between the microphone and the speaker and,
as a result, the system becomes somewhat cumbersome physically. As
a practical matter, it was also necessary for the user to adjust
the position of the various parts of the system as well as the
volume on it. For persons without technical ability, successful
operation of such a system was a hit or miss proposition and, in
practice, even a reasonable facsimile of the best possible
performance of the system was seldom achieved, with most users
settling for barely operational configurations despite various
electronic systems for attempting to alleviate these problems.
Another approach to this problem and one which is probably most
widespread in modern communication systems is the sacrifice of
duplex operation to trouble-free speaker telephone operation.
Generally, these systems incorporate an electronic switch which
either turns off the speaker when the user is speaking or disables
the microphone when the party at the other end of the telephone is
speaking and compares signal intensities when signals are being
produced at both ends of the telephone conversation.
In accordance with so-called "hybrid" technology, a duplex solution
to the speaker telephone problem, without the above difficulty, has
been approached. Generally, such systems operate by introducing a
hybrid electronic circuit which, is meant to approximate the
complex impedance of the telephone system, and to produce a
cancellation signal.
This cancellation signal, when added to the signal on the telephone
system (comprising both the transmitted and received signal)
results in generating a third signal which includes only the
received signal, which third signal is, in turn, sent to the
amplifier and loudspeaker of the speaker telephone system.
While such an approach would appear to provide a perfect solution
to the duplex speaker telephone problem, as a matter of fact, the
approach suffers from several inadequacies. Firstly, telephone
system line impedances vary greatly from system to system in
different parts of the country and even from exchange to exchange
within the same city. Thus, it becomes necessary for the system to
be installed and the complex impedance adjusted to minimize
feedthrough of the signal to be transmitted into the telephone
speaker amplifier. Naturally, this represents a substantial expense
insofar as it involves having a technician on site for installation
of the system. The increase in cost is significant enough that, for
the great majority of users, such systems are not, from an economic
standpoint, a practical option.
Moreover, even after such a system is installed, experience has
shown that the complex impedance of the telephone lines will vary
from call to call and from time to time depending upon the lines
being used by the central office switching system, environmental
factors, and the like. Thus, the above on site adjusted systems, at
best, represent only an approximation and, for that matter, an
approximation of irregular quality depending upon the nature of the
particular telephone system with which they are used.
SUMMARY OF THE INVENTION
The invention as claimed is intended to provide a remedy. It solves
the problem of how to provide duplex communication over
conventional telephone lines with good audio characteristics and
substantially without the above discussed problems. The same is
achieved through use of a multiple tone measurement and adjustment
sequence o the initiation of a telephone call either initiated at
or received by the inventive telephone system.
More particularly this is achieved by applying a plurality of tones
of different frequency to the telephone line, measuring the complex
received signal and then adjusting a synthesized RLC hybrid circuit
to more closely respond the same way as the telephone line and then
repeating that process, until an acceptable degree of conformity
between the hybrid and the telephone line has been achieved.
A speaker telephone, comprising a microphone and a loudspeaker is
disclosed. An amplifier has its output coupled to the loudspeaker.
This amplifier has non-inverting and inverting inputs. A switch
having two positions couples the microphone to the amplifier or
couples the tuning signal source to one of the inputs of the
amplifier. A tuning signal source and control circuit output a
tuning signal. Initiation means initiates a tuning sequence by
causing the switch to be in the second position and causing the
tuning signal source and control circuit to generate tuning
signals. A coupling device is connected to one of the inputs of the
amplifier and adapted to be connected to a telephone circuit. The
coupling device inputs signals from a telephone circuit to the
amplifier to be amplified by the amplifier and converted into audio
signals by the loudspeaker and couples signals from the telephone
circuit to the amplifier. A hybrid voltage generator circuit is
responsive to the output of the microphone when the switch couples
the microphone to the amplifier to generate at its output a
cancellation signal. The cancellation signal is coupled to the
other input of the amplifier. The cancellation signal has a
magnitude and phase which results in substantially cancelling at
least a portion of the signals produced by the microphone from
appearing at the output of the amplifier.
A detector responds to the output of the amplifier. In response to
the tuning signals, it generates an error signal. The circuit means
receives the error signal and derives control signals in response
thereto. The control signals are sent to the hybrid circuit to
control parameters of the hybrid circuit to achieve cancellation of
a substantial portion of the signal output by the microphone at the
output of the amplifier.
BRIEF DESCRIPTION OF THE DRAWINGS
One way of carrying out the invention is described in detail below
with reference to drawings in which:
FIG. 1 is a graphic representation showing impedance comprising the
vector sum of a reactance and a resistance;
FIGS. 2 and 3 illustrate the characteristics of a telephone
line;
FIG. 4 is a simplified block diagram of a speaker telephone system
constructed in accordance with the present invention;
FIG. 5 is a more detailed diagram of a telephone system constructed
in accordance with the present invention;
FIG. 6 is a diagram of a tuning signal used in accordance with the
system of the present invention;
FIGS. 7-13 illustrate operation of a part of a system constructed
in accordance with the present invention;
FIG. 14 is a detailed diagram of a portion of the circuit
diagrammed in FIG. 5; and
FIGS. 15-19 illustrate the characteristics of the circuit
illustrated in FIG. 14.
BEST MODE FOR CARRYING OUT THE INVENTION
Some idea of the complexity of the problem of generating an
effective hybrid circuit can be appreciated when one considers the
range of overall impedances which must be accommodated by the
system. It has been reported that in one series of measurements
from a single location involving calls to the same local exchange,
calls to other local exchanges, calls to suburban exchanges and
long distance calls to different area codes, those measurements in
the range of .+-.1 standard deviation at 200 Hz yielded results
varying from 450 to 1700 ohms. Thus, the 600 ohm impedance
routinely referred to in the literature as the impedance of a
telephone line is, from a practical standpoint, of minimal
importance to many applications.
Moreover, while the purely resistive component of the impedance of
a telephone line is for a given telephone call relatively well
behaved, the reactive component includes both capacitive and
inductive components. This results in the net impedance varying
with frequency both in magnitude (resistance plus reactance) and in
sign (inductive reactance or capacitive reactance).
The net inductive/capacitive component is commonly referred to as
the imaginary portion of the impedance or the reactance while the
resistive component of the impedance is referred to as the real
component. Referring to FIG. 1, the real component R and an
imaginary component X are vectorially illustrated. The resultant
impedance Z may be obtained by vector addition as is graphically
illustrated in FIG. 1. In the case of FIG. 1, the reactance is of a
capacitive nature and, accordingly, is referred to as X.sub.c. If
the reactance were of opposite sign (i.e., positive), the reactance
would be of an inductive nature and could be referred to as
X.sub.L. As an alternative to being expressed in terms of a real
and an imaginary component of a given sign, the impedance may be
viewed in terms of a magnitude Z and a phase angle .theta..
As noted above, the reactance of a telephone line varies as a
function of frequency. A typical curve showing the variation in
impedance Z as a function of frequency in a telephone line is
illustrated in FIG. 2. Likewise, the angular component for the sam
telephone line is illustrated in FIG. 3. As can be seen from these
figures, the magnitude Z and angle .theta. of the impedance
undergoes significant variation within a nominal working range of
200 to 5,000 hertz.
Referring to FIG. 4, a speaker telephone 10, constructed in
accordance with the present invention is illustrated. Generally,
speaker telephone 10 comprises one or more microphones 12 for
receiving an audio signal to be transmitted. The amplified output
voltage of microphones 12 are coupled to a hybrid balance circuit
14 and to a voltage dividing network 16 comprising a voltage
divider resistor 18 and the primary 20 of an isolation transformer
22 which couples telephone 10 to telephone lines 24. Transformer 22
is a 1:1 transformer of conventional design.
The output point 26 of network 16 is coupled to the inverting input
of a differential amplifier 28. The voltage output of hybrid
balance circuit 14 is coupled to the non-inverting input of
differential amplifier 28. The output of differential amplifier 28
is, in turn, coupled to a loudspeaker 30. During operation, a sound
32 to be transmitted impinges upon microphones 12 resulting in the
generation of an electrical signal proportional to the amplitude of
sound waves 32. This electrical signal is sent via resistor 18 to
the inverting input of differential amplifier 28.
In similar fashion, the output of microphones 12 is also sent to
hybrid balance circuit 14 which outputs a signal on its output line
34 which is equal in magnitude and phase to the signal produced at
point 26 by that portion of the signal from microphone 12 coupled
via resistor 18. Accordingly, the signal sent to amplifier 28 by
microphones 12 is subtracted from the signal produced over line 34
resulting in no output from speaker 30 in response to the output of
microphones 12. Thus, sound waves 32 which appear at microphones 12
do not exit the system through the speaker 30 of speaker telephone
10, to the extent that the signal on line 34 is identical to the
signal at point 26.
Conversely, the signal present at point 26 is transmitted via
isolation transformer 22 to the telephone line 24 for transmission
to the party with whom communication is occurring. Likewise,
signals received along line 24 are brought by isolation transformer
22 to point 26, where they are amplified and appear at loudspeaker
30 as an amplified received sound wave signal 36.
Dynamic hybrid balance circuit 14 is an intelligent complex (i.e.
frequency dependent) voltage divider designed to mimic the voltage
produced by the complex voltage divider comprising resistor 18 and
the complex impedance 38 of telephone line 24. This is achieved
during an initial set-up, when a switch 40 couples a test signal to
amplifier 28 from dynamic hybrid balance circuit 14.
FIG. 5 is a more detailed embodiment which illustrates this aspect
of the invention which allows automatic compensation for variations
in line impedance due to connection to different exchanges or the
employment of different equipment in the same exchange.
A detailed block diagram of a system constructed in accordance with
the invention is illustrated in FIG. 5. Generally, similar parts or
parts performing analogous corresponding or identical functions to
those of the FIG. 4 embodiment are numbered herein with numbers
which differ from those of the earlier embodiment by multiples of
one hundred.
Referring to FIG. 5, speaker telephone system 110 includes a switch
140 which couples tones from digital-to-analog converter 142 to
amplifier 115, in order to measure the telephone line impedance 138
and couples the output of microphone 112 to amplifier 115 during
normal voice operation of the speaker telephone.
Digital-to-analog converter 142 and synchronous detector 144 are
controlled by a digital processor 148. The analog output of
synchronous detector 144 sends the real and imaginary components of
the voltage at point 127 to an analog-to-digital converter 146.
Analog-to-digital converter 146 converts these voltages to digital
representations of their values and sends this digital information
to digital processor 148.
The output of digital processor 148 is sent to a pair of lines 150
and 152 which carry up/down control signals and increment control
signals, respectively. These two control signals adjust the
parameters of the hybrid voltage generator circuit 154. Hybrid
circuit 154 includes a plurality of adjustable resistances 160-168
which, in response to an increment command will adjust themselves
either upward or downward depending upon the state of their up/down
input. This has the result of adjusting that characteristic of the
circuit which is introduced into hybrid circuit 154 by networks
170-178, which are associated with adjustable resistances 160-168,
respectively. Network 170 comprises a simple resistive component.
Networks 172 and 174 are low pass RC networks which function to
introduce capacitive components. Networks 176 and 178 are high pass
RC circuits which introduce capacitive components into the
characteristic of hybrid circuit 154.
In accordance with the preferred embodiment, when a call is
received or initiated by the system by either answering of the
telephone or connection to an outside line, switch 140 is actuated
into the position shown in solid lines in FIG. 5, after which an
initial set-up sequence is followed. Digital processor 148 then
generates digital signals which cause digital-to-analog converter
142 to produce at its output sine waves at approximately 300 Hertz,
480 Hertz, 780 Hertz, 1260 Hertz, 2040 Hertz and 3300 Hertz, as
illustrated in FIG. 6. These frequencies are impressed onto the
input of amplifier 115 which in turn sends them to hybrid circuit
154 and, via resistor 118 to point 126.
The objective of hybrid circuit 154 is ideally to generate a
voltage on line 134 at the non-inverting input of amplifier 128, in
order that the voltage created at point 126 (in response to the
output of converter 142 during initial setups) will be subtracted
equally in magnitude and in phase in order that substantial
cancellation will occur in amplifier 128. To the extent that this
is achieved, the signal input into amplifier 115 will not appear at
the audio output 136 of speaker 130. In order for this to occur,
hybrid circuit 154 should have the same reactive and resistive
components as the complex impedance 138 of the telephone line. In
accordance with the invention, this is achieved by the adjustment
of resistances 160-168 by digital processor 148 in successive
interactions comprising application of sine waves by converter 142
of the above series of sine waves, detection of the voltage at
point 12 by detector 144 to produce an error signal, the furnishing
of this error signal by detector 144 to digital processor 148 and
the adjustment of resistances 160-168 by processor 148, and the
repetition of this cycle. Such repetition may be done a fixed
number of times (i.e., ten times) or until a desired degree of
matching is achieved between telephone line impedance 138 and the
impedance of hybrid circuit 154.
This is achieved by applying the output of amplifier 128 to
synchronous detector 144 and measuring the results and through
successive applications of the above series of sine waves and
successive adjustment of adjustable resistances 160-168 to match
the impedance 138 of telephone line 124 as reflected by 1:1
transformer 122 to point 126.
In order to achieve this end, during the set-up period the sine
wave tones from digital-to-analog converter 142 are coupled through
electronic switch 140 to amplifier 115, and from there to hybrid
circuit 154 and to point 126 via resistor 118. The response to
these tones created by the complex (i.e., frequency dependent)
voltage divider comprising line impedance 138 and resistor 118 are
subtracted from the synthesized approximation of these voltages
produced by hybrid circuit 134 and applied via line 134 to the
non-inverting input of amplifier 128.
If substantially zero voltage is present at the output of amplifier
128 for all frequencies, then a reasonable approximation of the
situation where the characteristics of the line have been matched
within an acceptable limit is present. The initial set-up testing
sequence is then terminated, and switch 140 switched to the
position shown in dashed lines in FIG. 5, allowing the telephone
conversation to begin. If desired, this can be signalled to the
telephone user by a light or tone.
Digital processor 148 determines whether the output of amplifier
128 is above or below an acceptable threshold level in the
following way. The output of amplifier 128 is applied to the input
of synchronous detector 144. Synchronous detector 144 receives two
digital control signals R.sub.phs and I.sub.phs from digital
processor 148 which enable the detector to resolve the uncancelled
sine wave at its input into two analog voltages V.sub.real and
V.sub.imag. These two voltages represent the extent to which hybrid
circuit 154 is not replicating the circuit comprising resistor 118
and telephone line impedance 138.
Depending upon the magnitude of the real and imaginary components
provided by synchronous detector 144, digital processor 148 adjusts
adjustable resistances 160-168 in order that a desired portion of
the characteristic created by networks 170-178 will change the
output of the hybrid circuit 154 by a desired magnitude that will
result in an appropriate cancellation signal at the non-inverting
input to amplifier 128.
After this initial adjustment, the sequence of the six tones
referred to above may be repeated by digital processor 148 and
digital-to-analog converter 142 and, if the digital processor which
functions as a threshold detector finds all signal frequencies
present at point 127 to be acceptably low for all of the tones,
digital processor 148 terminates the set-up sequence and restores
switch 140 to the position shown in phantom lines in FIG. 5. Again,
if the threshold detector finds the level of signal magnitude at
point 127 unacceptably high, the adjustment procedure is repeated
again.
Considering the operation of the inventive system in greater
detail, simultaneous with the generation of each of the sine wave
signals by digital-to-analog converter 142, respectively, digital
processor 148 generates the parameters R.sub.phs and I.sub.phs.
R.sub.phs is illustrated in FIG. 7 and I.sub.phs is illustrated in
FIG. 8. These are simple digital signals which go on and off in
phase and 90.degree. out of phase with the sine wave outputs of
digital-to-analog converter 142.
Synchronous detector 144 then derives two signals. In the first
case, the voltage at point 127 is coupled to synchronous detector
144 and the product of the voltage at point 127 and R.sub.phs is
generated. A possible signal at point 127 is illustrated in FIG. 9.
The product of that signal and the signal illustrated in FIG. 7
(R.sub.phs) is illustrated in FIG. 10. The signal is then
integrated over time to obtain the real component of the voltage at
point 127 as illustrated in FIG. 11.
In the second case, in a manner similar to that described above,
synchronous detector 144 generates the product of the voltage at
point 127 and the signal illustrated in FIG. 8 (I.sub.phs) to
generate the waveform illustrated in FIG. 12. This waveform is then
integrated over time and the resultant signal, illustrated in FIG.
13, has a value substantially equal to the magnitude of the
imaginary component of signal present at point 127. We can refer to
the signal present at point 127 as the hybrid voltage.
Depending upon the amplitude of the real and imaginary components
of the hybrid voltage, appropriate adjustments are sent along lines
150 and 152 by digital processor 148.
In accordance with the preferred embodiment of the invention, the
hybrid voltage generating circuit 154 takes the form illustrated in
detail in FIG. 14. More particularly, amplifier 115 is provided
with a pair of resistances giving it a net gain of 7.8, as
illustrated. Generally, it is noted that the values of principal
component parts illustrated in FIG. 14 are given directly on the
diagram for purposes of ease of understanding.
Adjustable resistances 160-168 each comprise digitally controlled
electronic potentiometers 190-198, respectively, which are
commercially available under Catalog No. X90103P from the Xicor
Company of Milpitas, Calif. The wiper terminals of potentiometers
190-198 are coupled via resistors 100-108, which have a value of 33
kilohms, to the non-inverting input of amplifier 128. Resistor 186,
which has a value of 390 kilohms, serves the function, in
conjunction with resistors 200-208, of providing an input impedance
at the non-inverting input of amplifier 128 that is equal to the
input impedance of all resistors at the inverting input of
amplifier 128. This equality assures cancellation of identical
input currents to balanced differential amplifier 128.
The outputs of networks 170-178 are, respectively, coupled to
potentiometers 190-198 via resistor 201 and amplifiers 203-209,
respectively. Amplifiers 203-209 have gains of 1.1, 11, 1.1, and
11, respectively, as a consequence of the values of the resistors
210-224.
The output of amplifier 203 is coupled via 10 kilohm resistor 226
which performs the function of scaling the voltage to potentiometer
192.
Capacitor 228 is used to couple the output of amplifier 204 to
potentiometer 194 through resistor 230. Capacitor 228 has a value
of 0.1 .mu.F, which means that it substantially passes all
frequencies above 100 Hz, but isolates potentiometer 194 from any
d.c. voltage. This is important because the gain of amplifier 204
can multiply the effects of any small d.c. voltage at the output of
amplifier 115.
Resistor 232 couples the output of amplifier 207 to potentiometer
196. There is no concern with respect to d.c. levels here because
capacitor 234 in network 176 removes any d.c. coupling. Similarly,
capacitor 238 prevents any d.c. levels from passing to amplifier
209 and appearing at the output of the amplifier.
It is noted that d.c. voltage present at the output of amplifier
203 is not a consideration because of the small gain of this
amplifier (i.e., about 1.1).
The outputs of amplifiers 203-209 are coupled via 100 kilohm
resistors 242-248 to the inverting input of amplifier 128 for the
purpose of providing both a positive and a negative adjustment
capability for electronic potentiometers 190-198.
In accordance with the preferred embodiment, the output of
amplifier 203 is coupled via another resistor 252 having a
resistance of 150 kilohms to the negative input of amplifier 205
for the purpose of making the real and the imaginary compensations
provided by networks 172 and 174 more independent.
Generally, in accordance with the present invention, potentiometers
192 and 196 are set at a position where the resistance between
their wiper terminals connected respectively to resistors 202 and
20 and ground is approximately one-third the total resistance of
the potentiometer along the path extending between ground and
resistors 242 and 246. The total resistance of each potentiometer
190-198 is approximately 10 kilohms. The position of the wiper
terminals of potentiometers 192 and 196 are adjusted in response to
the real and imaginary components generated at the output of
synchronous detector 144. Adjustment is made by digital processor
148 which outputs an up or down signal to the U/D inputs of
potentiometers 190-198 and a number of increments to the INC inputs
of the potentiometers in order to bring them to the desired
value.
For example, if the real component is found to be relatively large
at 780 and 1260 Hz and positive potentiometer 190 will be
decremented a scaled number of pulses by digital processor 148.
On the other hand, if the real component is found to be relatively
large only at 200 Hz and 480 Hz and negative, potentiometer 192
will be incremented up a scaled number of pulses by digital
processor 148.
If the imaginary component is found to be relatively large at 300
Hz and 480 Hz and positive, potentiometer 196 will be decremented a
scaled number of pulses by digital processor 148.
Further, if the imaginary component is found to be relatively large
at 2040 and 3300 Hz and negative, potentiometer 198 will be
decremented a scaled number of pulses by digital processor 148.
Finally, if the imaginary component is found to be relatively large
at 2040 Hz and 3300 Hz and negative, potentiometer 198 will be
decremented a scaled number of pulses by digital processor 148.
The operation of the circuit illustrated in FIG. 14 may
alternatively be understood with reference to FIGS. 15-19. In
particular, as illustrated in FIG. 15, adjustment of potentiometer
190 has the primary effect of varying the resistive component. This
will affect the voltage at point 127 as illustrated graphically in
FIG. 15. The degree to which real components are scaled by
adjustment of potentiometer 190 is illustrated in FIG. 15 in terms
of millivolts per increment at analog-to-digital converter 146.
In accordance with the preferred embodiment, the resistance and
capacitive reactance of that portion of the hybrid circuit
comprising network 172 and electronically adjustable resistance 162
are of comparable magnitude at the lowest frequency of about 300
Hertz. See FIG. 16. Reactive capacitance decreases by an order of
magnitude at 3300 Hertz, as does the other component. Accordingly,
this circuit tends to compensate for line impedances of the type
which are typically caused by fixed inductances at specific points
in the lines. Sometimes these are referred to as lumped
impedances.
Generally, combinations of the various scaled amounts of the five
characteristic curves (FIGS. 15-19) enable approximate modeling of
the various frequency dependent impedances (i.e., lumped and
distributed resistance, inductance and capacitance of most
telephone lines. FIGS. 15-19 correspond to the effects in
millivolts per increment of the adjustment of potentiometers
190-198 on the real and imaginary characteristics of the
system.
To summarize, FIG. 15 shows the variation of the hybrid resistive
component by adjustment of potentiometer 190. FIG. 16 shows the
variation of the hybrid low frequency real and imaginary components
by adjustment of potentiometer 192. FIG. 17 shows the variation of
the hybrid low frequency imaginary component by adjustment of
potentiometer 194. FIG. 18 shows the variation of the hybrid high
frequency real and imaginary components by adjustment of
potentiometer 196. FIG. 19 shows the variation of the hybrid high
frequency imaginary component by adjustment of potentiometer
198.
At the lowest frequency, the portion of hybrid circuit 154 composed
primarily of network 174 and adjustable resistance 164 has a
reactive capacitance less than its real component at 300 Hertz.
Typically, the real component would be an order of magnitude
greater than the imaginary component. At 3300 Hertz, both terms
become negligible. See FIG. 17. It is noted that the gain of this
portion of the device is an order of magnitude greater than that of
the other capacitance circuit described above.
The circuits controlled by potentiometers 196 and 198 compensate
for distributed inductive effects and lumped inductive effects,
respectively.
While an illustrative embodiment of the invention has been
described above, it is, of course, understood that various
modifications will be apparent to those of ordinary skill in the
art. Such modifications are within the spirit and scope of the
invention, which is limited and defined only by the appended
claims.
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